The present invention is adapted to be used in a multiple ratio transmission situated in a vehicle drive line having an internal combustion engine and a hydraulic torque converter situated between the engine and an input shaft of the transmission.
A multiple ratio transmission is described in "RE4R01A TYPE AUTOMATIC TRANSMISSION SERVICE MANUAL" issued in 1987 by the assignee. This transmission is shown in U.S. Pat. No. 4,680,992, issued Jul. 21, 1987, to Hayasaki et al. This patent is assigned to the assignee of the present invention.
This transmission comprises, as its gearing elements, two simple planetary gear units arranged between the transmission input shaft and the transmission output shaft. The two simple planetary gear units include a rear planetary gear unit and a front planetary gear unit situated between the rear planetary gear unit and a hydraulic torque converter. The rear planetary gear unit includes a rear sun gear connected to the input shaft, a rear planet carrier connected to the output shaft, and a rear ring gear. The front planetary gear unit includes a front sun gear, a front planet carrier, and a front ring gear connected to the rear planet carrier. A forward clutch interconnects a low one-way clutch and a forward one-way clutch. Upon engagement of the forward clutch, the low one-way clutch and the forward one-way clutch are brought into cooperation with each other. They overrun in the engine rotational direction and effect a torque reaction in the opposite rotational direction, thus allowing the rear planet carrier to rotate in the engine rotational direction, but preventing its rotation in the opposite rotational direction. The low one-way clutch overruns in the engine rotational direction and effects a torque reaction in the opposite rotational direction, thus allowing the front planet carrier to rotate in the engine rotational direction, but preventing its rotation in the opposite rotational direction. An intermediate ratio brake is adapted to anchor the front sun gear to establish a torque reaction flow path associated with an intermediate speed ratio (=second gear) and also an overdrive (=fourth gear). A high ratio clutch is adapted to interconnect the rear planet carrier and the input shaft. A reverse clutch is adapted to interconnect the front sun gear to the input shaft. A low reverse brake is adapted to anchor the front planet carrier. This known transmission has four speed ratios in Drive and a reverse. A low speed ratio (=first gear) in Drive is established by engagement of the forward clutch. For a shift from low speed ratio to intermediate speed ratio in Drive, the intermediate ratio brake is engaged with the forward clutch engaged. For a shift from intermediate speed ratio to a high speed ratio (=third gear) in Drive, the intermediate ratio brake is released and the high ratio clutch is engaged with the forward clutch engaged. For a shift from high speed ratio to overdrive (=fourth gear) in Drive, the intermediate ratio brake is engaged with the forward and high ratio clutch engaged. For a downshift from high speed ratio to intermediate speed ratio, the high ratio clutch is released and the intermediate ratio brake is engaged.
The intermediate ratio brake includes a fluid pressure operated intermediate servo. The intermediate servo has a double acting piston defining an intermediate brake apply pressure chamber and an intermediate brake release pressure chamber. The double acting piston has an integral auxiliary piston defining a second intermediate brake apply pressure chamber. A pressure accumulator communicates with the intermediate brake release pressure chamber and the high ratio clutch.
Ratio change between intermediate speed ratio and high ratio on an upshift is achieved by controlling pressure build-up within the intermediate brake release pressure chamber and pressure build-up within the high ratio clutch. Ratio change between the high ratio and the intermediate ratio on a downshift is achieved by controlling pressure reduction within the high ratio clutch and pressure reduction within the intermediate brake release pressure chamber.
FIG. 10 is a simplified illustration of a 3-2 downshift control system of the transmission for effecting a downshift from high speed ratio to intermediate speed ratio by controlling discharge of oil from the intermediate brake release pressure chamber and the high ratio clutch.
In response to a command for a 3-2 downshift from the high speed ratio to intermediate speed ratio, a shift solenoid triggers movement of a shift valve. This initiates discharge of oil from the intermediate brake release pressure chamber and the accumulator 323 chamber through calibrated flow control orifices 322 and 321 and discharge of oil from the high ratio clutch H/C through the calibrated control orifice 321. The intermediate brake release pressure P.sub.3R and the high ratio clutch apply pressure P.sub.HC drop at rates that are determined by the accumulator 323, the double acting piston of the intermediate servo, and the orifices 321 and 322.
FIG. 11 is a 3-2 downshift timing diagram illustrating variations of the high clutch apply pressure P.sub.HC and the intermediate brake release pressure P.sub.3R initiated by the command for a 3-2 downshift. Immediately after a drop in oil pressure at moment t.sub.1, the double action piston of the intermediate brake servo begins to move in such a direction as to decrease the volume of the intermediate brake release pressure chamber. Concurrently, the piston of the accumulator 323 begins to move in such a direction as to decrease the volume of the accumulator chamber. At moment t.sub.2, the double action piston comes to an end of its movement and the engagement of the brake B/B begins. During the time period t.sub.1 -t.sub.2, the pressures P.sub.3R and P.sub.HC drop at ramp rates that are determined by the movements of the double action piston and the piston of the accumulator 323.
After the moment t.sub.2, the movement of the piston of the accumulator 323 continues and the pressures P.sub.3R and P.sub.HC reduce at ramp rates that are determined by the accumulator 323. At moment t.sub.4, the pressures P.sub.3R and P.sub.HC are reduced to zero.
The calibrated flow control orifice 322 has an orifice size that is unaltered over the time period t.sub.1 -t.sub.4. This orifice size determines the apply timing of the intermediate brake B/B (at moment t.sub.2).
With regard to the intermediate ratio brake B/B, its torque transmitting capacity QBB develops from the moment t.sub.2 before the completion of inertia phase (at the moment t.sub.3). The inertia phase is completed when engine speed Ne reaches a level corresponding to the intermediate speed ratio.
With regard to the high ratio clutch H/C, its torque transmitting capacity Q.sub.HC remains at a considerable level over the time period t.sub.2 -t.sub.3, resulting in creation of interlocking tendency of the high ratio clutch H/C and the intermediate brake B/B. This explains an increased rate at which the transmission output shaft torque T.sub.o drops over the time period t.sub.2 -t.sub.3, as illustrated within a portion encircled by a phantom line circle .alpha.1. This causes increased torque disturbance.
At the moment t.sub.2, the torque transmitting capacity Q.sub.HC drops down to a level, which is too low to hold dragging force through the high clutch H/C at a sufficiently high level Thus, the dragging force is not strong enough to suppress the magnitude of an increase, as illustrated within a portion encircled by a phantom line circle .beta.1, in transmission output torque To that occurs at the moment t.sub.3. The reduction of magnitude of this increase in transmission output shaft torque is needed to improve shift feel at the moment t.sub.3.
U.S. Pat. No. 4,709,596 (.noteq.JP-B 6-58145) teaches independent discharge circuits for a high ratio clutch and an intermediate brake for a downshift control system from a high ratio to an intermediate ratio. The discharge circuit for an intermediate ratio brake includes a timing valve. This known control strategy is described along with FIG. 12.
FIG. 12 is a 3-2 downshift timing diagram illustrating variations of the torque transmitting capacity Q.sub.3 of the high ratio clutch and the torque transmitting capacity Q.sub.2 of the intermediate ratio brake. The known control strategy aims at rapid increase of the torque transmitting capacity Q.sub.3 of the intermediate ratio brake, at moment t.sub.3, upon the completion of inertia phase. To accomplish this aim, it is proposed to open the timing valve at a moment t.sub.v before the moment t.sub.3 by a time interval .DELTA.t, to increase apply rate of the intermediate brake. This is effective in eliminating the interlocking tendency, thus eliminating rapid drop of transmission output torque near the completion of inertia phase. However, this known control strategy fails to suppress the magnitude of increase .beta.1 in transmission output torque To that takes place upon the completion of inertia phase (at moment t.sub.3).